Induced endothelial cells enhance osteogenesis and vascularization of mesenchymal stem cells

Adequate vascularization remains one of the major challenges in bone tissue engineering. Since the microvascular endothelium is of benefit to osteogenesis and vascularization when in direct contact with bone marrow mesenchymal stem cells (BM-MSCs), we investigated whether endothelial cells induced from BM-MSCs have the same effect on BM-MSCs in vitro and in vivo. BM-MSCs were isolated, characterized and induced into endothelial-like cells (induced endothelial cells, IECs) in endothelial cell growth medium 2. BM-MSCs and IECs were co-cultured with direct contact. In vitro, IECs were evaluated in terms of their characteristics of endothelial cells and their effects on the osteogenic potential of BM-MSCs by cell morphology, immunofluorescent staining, alkaline phosphatase activity and osteocalcin synthesis. In vivo, scaffolds consisting of beta-tricalcium phosphate co-seeded with IECs and BM-MSCs were transplanted into mouse dorsal pockets, and a histological analysis was performed to determine the extent of new bone and blood vessel formation. Isolated BM-MSCs were positive for the markers CD105 and CD29 and negative for hematopoietic markers CD34, CD45 and CD14. They were able to differentiate into adipocytes, osteocytes and chondrocytes in respective media. Immunofluorescent analysis with von Willebrand factor and CD31 staining showed that BM-MSCs could differentiate into endothelial cells. The alkaline phosphatase activity and the osteocalcin content of the co-culture group were obviously higher than those of any other group (p < 0.05). Histologically, newly formed bone and vessels were more evident in the culture group (p < 0.05). Our findings suggest that IECs could efficiently stimulate the in vitro differentiation of osteoblast-like cells and promote osteogenesis in vivo by direct contact with BM-MSCs.